I don't get why the 86 gets a pass but the Supra doesn't. Other than the trans and D4-S, it's a Subaru. It uses a Subaru engine, a Subaru chassis, and is built in a Subaru factory. Why is okay for Toyota to co-develop a car with Subaru, but not BMW?

By this logic the MKV is spiritually similar to the MKIV. Turbo i6, RWD, coupe, and very tunable. Its unique mark is that instead of being a GT with a killer engine, it's a legit sports car with a killer engine.

My best attempt to understand why the Supra is more heavily criticized is this... (and keep in mind this is heavily centered around the USA)

I think it starts with the GT86 platform being only a spiritual successor of the original AE86. It's not a new generation of the Corolla going back to those roots, but an entire new platform co-opted with Subaru. The MKV isn't. It directly revives the 'Supra' nameplate and officially goes into the history books as the fifth generation. So there's like an insane expectation that this thing has to top the MKIV, which the legacy, as you well know, has been building for decades. The Supra (at least in the USA) has had a significantly larger cultural impact and popularity in the car scene, so given the hype, it was an impossible feat for Toyota to achieve realistically.

I think some people just expected a mostly Toyota built successor with a 'next gen' 2jz engine that would do unnatural things with a big ass turbo, a 'stock bottom end', and super beefy manual transmission. I mean shoot, the new one may have an auto, but at least it's still a German transmission!

I kid, but really the MKV was just never going to live to the hype in some of the fan boy's eyes. That just didn't exist as strongly or widely with the GT86.